Salt bath furnace



July 8,' 1941 J. D. GRIFFIN 2,248,474

SALT BATH FURNAGB Fiied Aug. 1, 1959 2 sheets-sheet 1 yi-:1.1. A C 23 Q0gli INVENToR Josep/22 @rf/jin Nw@ M ATTORNEYS July 8, 1941. J. D.GRIFFIN SALT BATH FURNACE Filed Aug. 1, 1939 2 Sheets-Sheet 2 lNvEN-roRJoseln Gry/2n,

ATTORNEYS Patented July 8 1941 SALT BATH FUBNACE Joseph Darius Grimn.Johannesburg, Transvaal, South Africa Application August .1, 1939.-Serial No. 287,725 In the Union of South Africa February 22, 193920Claims.

This invention relates to salt bath furnaces and particularly, thosearranged for heating the ends of rod-like bodies such as rock drillingtools and other ,bodies adapted to be suspended with a portion extendinginto the salt bath of the furnace.

Among the important objects of the present invention are to increase thecapacity of the furnace by permitting the continuous movement of aseries of articles through the bath; to provide for the properpreheating of the articles before entering the bath: to insure theimmersing of the articles to a predetermined and accurately controlledextent; to provide for the progressive increase in the temperature ofthe bath along the path of movement of the articles: to increase thevolumetric eiliciency of the bath; and to accomplish various otherobjects which will be apparent from the kdrawings and the followingdescription or which will be later pointed out.

In carrying out my invention, I provide means which defines the path fora series of yarticles and along which said articles may be moved whileimmersed to the desired extent in the bath so that the articles enterthe furnace at one part thereof, are heated to the desired temperatureduring movement along the path and emerge at the properly heatedtemperature. 'I'he bath is formed of a salt which is solid atatmospheric temperature and which is heated and kept molten as, forinstance, by the passage of an electric current flowing through thebath. The articles are preferably moved in a direction at right anglesto the direction of current ilow between the electrodes, and the heatingmay be to a different degree atl the 'part of the bath from which thearticles are withdrawn than it is in the part of the bath at which theyare introduced.

The electrodes may be spaced apart to a distance only slightly greaterthan thewidth of the articles to be heated so that the articles may movealong between the electrodes and these articles may be spaced veryclosely together so that the minimum size of bath may be employed fortreating the maximum number of articles in a given time. Two parallelbaths or bath sections may be formed by the use of three electrodes, orby only two electrodes if one consti- Preferably, arrangement is madewhereby the series of articles pass through the bath at a constant speedsuch `that the time period within which each remains in the bath is thatrequired to bring it to the desired temperature. As the temperature ofthe article will not exceed the temperature of the bath, and as thetemperature of the bath may be readily maintained constant, it will beapparentsthat a slower movement of the articles does not result in anyoverheating. By continuously immersing fresh articles to the bath at theinlet end of the path and simultaneously discharging heated articlestutes the wall of the receptacle for the salt bath,

so that the potential in the two paths or path sectionsmay be maintainedequal and thus, if the articles are electrically conductive, it is notnecessary that they be insulated or even separated from one another inthe line of travel or between the two path sections.

from the other end of the path, and by having the articles closelyspaced, the useful throughput of the bath is greatly increased while thecost of operation and upkeep of the furnace per article drops almostproportionally to the increase ln the lnumber of articles immersedsimultaneously because only a small portion of the heat taken up by thesalts isutilized in heating the metal parts immersed within vthe bath.

Furthermore, the uniform loading of the bath makes it unnecessary toprovide a large depth of salts below the point to which the articles areimmersed, and which has heretofore been considered necessary for thestorage of heat and to avoid temperature fluctuations with variations inworking conditions.

Although the present invention may be employed for the heating ofvarious articles, certain features of the invention are of specialimportance in connection with the heating of rock drill tools whichordinarily have a longitudinal passage therethrough. Upon the insertionof the end of such a rock drill tool directly from atmospherictemperature into the molten bath, thev air in the central passage isheated extremely rapidly and in rushing out from the upper end of thetool creates a suction which draws the molten salt up into the passagewhere it may solidify or may even spurt from the upper end of the toolwith liability of serious burning of operators.

As a feature of my invention, the tools before immersion in the moltensalt are temporarily held above the level of the salt or in an adjacentextension of the furnace, so that they become .rapidly preheated fromthe air or gases in or blowing out of the furnace and the closeproximity 'to the bath and therefore, when they do enter the bath, theair within the central passage has already become heated and expandedand there is no suction effect and no liability of drawing molten saltup into the passage.

As an important feature of the inventionI there is provided a conveyingmeans which moves the articles along the predetermined path and whichlowers them into the bath to the predetermined extent only after theyhave had a suiilcient time.

interval to become properly heated.

As a further important feature, there are provided spacers between thearticles to hold them in the proper spaced relationship and at the sametime to effectively close the top of the furnace and prevent escape ofheat. Preferably, the top of the furnace is spaced to some distanceabove the level of the bath, the articles or the suspending meanstherefor pass along a comparatively narrow slot in the roof which iseffectively closed by the spacers, and the movement of the articlesalong the bath is effected by a conveyor abve the roof which controlsthe elevation of the articles and not only moves the articles butthrough them, moves the spacers.

In the accompanying drawings, there are shown several different forms inwhich various features of my invention may be incorporated. In thesedrawings, A

Fig. 1 is a top plan view showing the parts somewhat conventionallyillustrating the parts and the rock drill tools being shown in section;

Fig. 2 is a vertical longitudinal section on the line 2-2 of Fig. 1;

Figs. 3 and 4 are transverse sections on the lines 3--3 and 4 4respectively-of Fig. 2;

Fig. 5 is a vertical section on the line 5--8 of Fig. 2;

Fig. 6 is a view somewhat similar to Fig. 2 but more diagrammaticallyshowing another form with one electrode on the bottom and the articlesserving as the other electrode;

Fig. 7 is a somewhat diagrammatic top plan view of an apparatus arrangedwith two parallel baths;

Fig. 8 is a vertical section through a furnace having two bath sections.

Fig. 9 is a top plan view of a further form in which there is a singleendless bath with the articles inserted and withdrawn at the same pointin the bath;

Fig. 10 is a somewhat diagrammatic view similar to a portion of Fig. 8but showing a wiring diagram and another means for connecting thecurrent to the electrodes;

Fig. ll is a sectional detailv showing how the electrodes may be mountedin the wall of the furnace;

Fig. 12 is ya top plan view of ,another type of furnace in which thepreheating feature of my invention may be carried out; and

Fig. 13 is a section on the line iii-I3 ofFig. 12.

In the specific form illustrated in Figs. 1 to 5 inclusive, there isprovided a furnace i0 adapted to hold the salts constituting the bath.The walls of the furnace are of heat insulating material and of acharacter which will not be adversely affected by the molten salts.These walls are shown only diagrammatically as it will be understoodthat in practice, they are very much thicker and of such thickness andwith such insulating spaces as to conserve the heat and prevent anysubstantial amount of radiation to the outside atmosphere.

'I'he chamber for the bath is formed of a length sufllcient to permit ofthe proper heating of the articles While traveling lengthwise thereofand is preferably of a width only slightly greater than the maximumwidth of the portions of the articles inserted in the bath. In this formthere are shown electrodes Il and I2 set into the inner surfaces of theopposite side walls and parallel to each other and parallel to thedirection of travel of the articles. Each electrode may, if desired,

be made up of separate sections as shown at il and Ila in Fig. 2 so thatthere will be the minimum liability of buckling or displacement due totemperature changes.

The electrodes may be mounted as shown in Fig. 11, that is, the bottomedge may be slightly tapered and rest against inclined surface Il sothat as the electrode progressively becomes thinner from corrosionduring use, it may be raised slightly from time to time and have itsface brought back to the proper plane and packing I la inserted in therear. Tov start with, the thickness of the electrode may be equal to thedepth of the recess and no packing or space for packing would bepresent.

I have not illustrated any means for efficiently heating the salts tobring them to the molten condition, but this may be done by anyconventional means commonly employed in connection with salt bathfurnaces. The alternating current of the desired potential and currentstrength may be conducted to the electrodes in any suitable manner asfor instance, by bus bars Il secured to flanges I5, extending outthrough opposite walls of the furnace.

The furnace is provided with a cover I6 which may be removed forcleaning, for adjustment of the electrodes or for other purposes. Thiscover has a slot I 1 in the top thereof which is of a width. onlyslightly greater than the diameter of the rods or the other suspendingmeans for the articles or parts to be heated. The ordinary rock drilltool has a shank A with a central passage B therethrough and a cuttinghead C at one end thereof and larger than the shank. For the treatmentof such articles, the slot i1 and the cover have enlarged portions I8 atthe ends thereof which are of sufficient size to permit the insertionand Withdrawal of the tools.

Above the cover I6 there is provided an endless conveyor I9 which may bein the form of a pair o f chains mounted on sprocket wheels 2li drivenin any suitable manner and at the desired speed. The upper runs of thetwo parallel chains serve to support the articles at the desiredelevation. The supporting means rwhich also provides for the desiredvertical positioning or adjustment of the articles may be in the form ofclips 2l which are secured to the Shanks of the drills, or onto othersuspending means with one end of each clip resting on one chain, and theother end resting upon the opposite chain. Each may carry a pair ofsemi-circular liner sections for directly engaging the shank and thesemay be of insulating material to prevent transfer of current to anydrill to another or from the conveyor mechanism. The clips arepreferably so formed that they directly contact one with another andthus hold the articles in the desired spacing.

The conveyor chains may be of any suitable construction but preferablyinclude rollers which are supported upon track members 22 so that theweight of the upper runs of the chains as weil as the articles aresupported by the track members. These track members may be supported inany suitable manner as for instance, by brackets 23 secured to theopposite walls of the furnace and vertically adjustable so as to varythe elevation of the brackets in respect to the top of the furnace orthe surface of the molten bath. In Fig. 2 this adjusting means is shownsomewhat diagrammatically as vertical slots in the base portions o! eachbracket and through which the clipping bolts are secured.

The upper side of each track is preferably arranged so that theelevation of the tools will be changed during the movement of thearticles along the bath. As shown in Fig. 2, the upper edge of the righthand portion of the track 22 is downwardly inclined for a short distanceso that when the article is inserted to the position controlled by thelocation of the clip thereon, the lower end of the article will beslightly above the liquid level of the bath. Thus. the lower endbecomems heated before contacting with the bath and the air inside ofthe lower end of the article may be likewise heated so that there willbe little or no further expansionof this portion of the air after thelower end of the central passage is closed by the entry of the articleinto the bath. The major portion of the upper edge of the track 22 issubstantially parallel with the liquid level so that the articles, asthey move along, are inserted to a predetermined andsubstantiallyuniform distance into the bath. If desired, the portion of the uppersurface of the track nearer the point of discharge for the articles maybe inclined upwardly so that the lower ends of the articles will bepartially withdrawn from the bath, or held close to the surface thereofduring the final portion of the movement to the point of discharge. Oneoperator may continuously feed successive articles through the openingnI8 at one end of the bath and at the rate at which the conveyor istraveling, while another operator may progressively move the articlesfrom the furnace at the other end of the bath.

In order to conserve the heat within the bath, there are provided aseries of spacers 25 which rest uponthe top of the cover I6 andeffectively close the slot l1 in the latter. 'I'hese spacers are of awidth substantially equal to the center line spacing of the articles andhave notches or recesses in opposite sides to form openings snuglyfitting the articles or the suspending means for the latter. Thus, theentire top of the slot is closed by the series of spacers and theoperator who feeds in the articles at one end applies a spacer to thecover for each article as he inserts it and supports it on the conveyor.As later pointed out, the spacers may form thecover so that no slotwould be employed.

As the articles are moved along by the lconf veyor, the spacers aresimultaneously slid along the cover, and as each article is removed thespacer adjacent theretomay drop into a receiver 26 at the end of thefurnace. 'I'hese spacers are from time to time taken around to the otherend of the furnace and one is put in place for each article inserted inthe bath. The spacers may be of any suitable material but preferably, ofheat insulating material, but' they do not need to have any very greatstrength because they do not carry any load and are merely slid along bythe articles. The spacers not only conserve the heat but prevent fumesor gases escaping from the top of the furnace where they might beinjurious to the workmen. To permit of the withdrawal of such fumes orgases, the furnace at one end may be provided with a gas outlet con-`duit 21 which` may extend to any point of disposal of the gases or maybe connected to a blower for continuously withdrawing such gases.

It is important that the level of the bathy be maintained substantiallyconstant. As the level elevation according to the number of articlesinserted, or the degree of insertion, or the size of the articles, thereis preferably provided an r`overflow chamber 28 into which excess liquidfrom the bath may overflow a sharp edge weir 2l, and further salt may becontinuously or intermittently fed to the bath to insure the maintenanceof the level up to the top of the weir.

It is desirable that the bath be maintained at a somewhat highertemperature near the point of discharge of the articles than at thepoint of insertion. This may be accomplished in various ways. As shownin Fig. 2, the electrodes ll and Ha are inclined at their lower ends sothat the electrode area per lineal foot along the length of thebath isgreater at the discharge end than it is at the entrance end. The largerthe immersed surface of the electrode at any part of the bath, thelarger is the amount of current which will be discharged through thebath to heat that part. Other means for accomplishing this result willbe pointed out hereinafter.

In order to impede convexion currents which might tend to keep the bathof uniform temperature at opposite ends, there may be Provided one ormore transverse partitions 30, the upper edges of which terminate belowthe level to which the articles are to be inserted. If the electrodes beeach formed of a series of sections, then these insulating transversepartitions may be disposed between adjacent sections of the electrodes.The electrodes are preferably spaced to some distance above the bottomof the furnace and this bottom may be inclined from one end to the otherso that slag which collects at the bottom of the furnace, either fromdirt or oxide coming in with the articles or from any other source, canbe accumulated at a low point from which it may be withdrawn by a scoopor any other convenient means. The bottom may also taper toward thecenter to form a narrow channel or trough 32 as shown in transversesection in Fig. 5 for the same purpose.

`As the salt removal by a plurality of articles from the bath will'tendtov lower the liquid level, particularly when the bath surface is verysmall at thetop, it is desirable to provide means for increasing thesurface area at the top of the bath so as to minimize such changes inliquid level. As shown in Fig. 4, the bath at one end and preferably atthe discharge end is provided with one or more laterally extendingsections 33. These may be of any desired shape and on either or bothsides, and of an area dependent upon the area of the surface of the bathalong the path of the articles and the extent to which the articles are`immersed in the bath. These lateral extensions do not need` to extenddown to the bottom of the bath, but may be only at the upper partthereof y Various different means may be employed for varying theheating effect at different points of'the bath so that the bath will bethe hottest at the point Where the articles are withdrawn. Also, variousdifferent constructions and arrangements of electrodes may be employed.In Fig. 6. there is shown a construction in which most of the partsaresubstantially asL above described, but there is provided an electrodelib which is inclined in respect `to the surface level of the bath andthe articles constitute'the other electrode. At the entrance end theelectrode is at the greatest distance from the articles and therefore,there will be a. higher resistance to current flow and a lower amount ofcurrent and a lesser heating effect than at the discharge end where theelectrode is comparatively close to the articles forming the otherelectrode. The current may be conducted to the articles in amr suitablemanner as for instance, by connection to the guide rail 22 which may bein relectrical connection with the articles through the conveyor andclips 2l.

It is not essential that the path be a straight one and there arecertain advantages in making the path annular so that one operator mayremove heated articles and insert fresh ones at the same point in thebath. In Fig. 9, I have shown a furnace Mc which has an annular slot Ilcwith a single enlarged portion or hole llc. In this case there may beprovided a central electrode lie and the wall of the furnace mayconstitute the other electrode. In order to keep the slot closed at allpoints except at the hole I 8c, the spacers 25e may be tapered radiallytoward the center of the curved portions of the path so that they willabut and not overlap in passing around the curve a'nd will keep the slotcovered.

In Fig. 7, there is shown somewhat diagrammatically a construction inwhich there are two parallel slots Hd so that two series of articles maybe advanced simultaneously. In such a construction, there may beprovided a pair of electrodes Ilcl and I 2d on the opposite walls and acentral electrode ii' dividing the bath into two portions as shown inFig. 8. By using a threephase circuit and connecting one conductor tothe central electrode and the other two conductors to the sideelectrodes, the potential across the two bath portions will bemaintained equal and it will not be necessary to insulate the articlesfrom each other, or from the conveyor or other parts.

It is not essential that the cover, as shown in Figs. 1 to 5 inclusiveand Fig. 7, be provided with slots. The entire cover in the form shownvin Figs. 1 to 5 inclusive may be formed by the spacers 25 which mayrest directly upon the furnace wall. This arrangement is shown in Fig. 8where the spacers 25d each have two notches in each side thereof insteadof only one and rest directly on the side walls. In this case thespacers may be made of stiffenand stronger material and may serve asthe'means for supporting the articles themselves. The clips 2Id whichare secured to the articles may rest directly upon the spacer sections25d and move along with them.

In the form shown in Figs. 7 and 8, the same general` type of conveyormechanism hereinbefore described may be employed with means for varying'the elevation of the articles at different points in the path. When thespacers support the articles to be heated, the surfaces on which thespacers rest must be varied in height relative to the salt level inorder to vary the elevation of the articles as these surfaces then servethe same purpose as the track members 22.

In Fig. 10 there is illustrated somewhat diagrammatically a constructionin which the spacers 25e constitute the top wall as in Fig. 8, but inwhich the articles serve as the electrodes. As the articles move along,the eiective delivery of current to the articles may be accomplished bythe use of troughs 40 along the sides of the furnace and containingmercury. The two troughs may be connected by conductors 4i to two of thewires of a three-phase circuit, and each article may have a clip 2Ieelectrically connected thereto and provided with a conductor I2extending into and movable along the trough of mercury so that the flowof current to the electrodes will be insured during the movement of theelectrodes through the bath.

'Ihe furnace may be provided with a central electrode 4I connected tothe other wire of the three-phase system so that the current will iiowbetween the central electrode and both rows oi' articles. This furnaceis also shown as provided with side electrodes as in Fis. 8 so that theconductors 4| might be connected to the side electrodes instead of tothe articles themselves.

Although my invention is particularly applicable for the simultaneousheating of a series oiarticles, it will be obvious that the apparatusmay be employed for heating of only one article at a time. Obviously,the heating etlect on each article will be the same and the apparatuswill operate the same with only one article in the bath as it will witha series. Even with only a single article being heated at one time, theapparatus has the advantage of delayed immersion and differenttemperatures at opposite ends of the path.

The delayed immersion feature of my invention may be employed inconnection with furnaces where the articles are not progressively movedthrough the bath. In Figs. 12 and i3 I have shown a furnace Il having amolten salt bath 48, a horizontal electrode 41 on the bottom, andelectrical connections to the articles A to be heated. The cover Il isshown as provided with a plurality of holes through which articles maybe inserted to the required distance and immersed to the desiredextent.- With such a furnace. one of the articles may be inserted,first, to a point slightly above the bath so that it becomes preheated,and after being held in that position for the required time, may beimmersed to the desired distance in the bath.

Such a small furnace might be used i'or heating a small number ofarticles so that when an article is removed from one hole in the cover,another one may be inserted, first, to the preheating point, and theninto the bath, and another one of the articles removed after beingheated and replaced by another. As many holes would be provided in theapparatus as required so that the articles may be removed in successionand each one heated to the proper temperature before removal.

Having thus described my invention, what I claim as new and desire tosecure vby Letters Patent is:

1. The method of heating th ends of rod-like bodies such as rock drilltools, which includes maintaining a salt bath by the direct passage ofelectric current through the bath material between spaced electrodes,vertically suspending the rod-like bodies with their lower ends immersedin the bath, and moving the series of bodies along a path such thattheir lower ends 1goltllow a substantially equipotential line in the 2.'I'he method of heating the cutting head of a hollow rock drill tool,which includes suspending the tool vertically with its head adjacent tobut spaced above a molten salt bath, and thereafter immersing the end ofthe drill in said bath to heat it to substantially the temperature ofthe bath, the spacing of the drill head in respect to the surface of themolten salt bath and the time during which the tool is so suspendedbeing such that the air within the lower end of the tool becomes heatedto a high temperature, greatly expanded, and flows out before said headis immersed, whereby the further heating of the air in said body, afterimmersion, does not cause such further expansion and upow of air in thebody as willsuck molten liquid from said bath up into the body to alevel substantially above the level of the bath.

3. The method of heating rod-like bodies such as rock drill tools bymeans of a salt bath furnace heated by the flow of electric currentthrough the bath material between parallel electrodes, which methodincludes immersing the ends of the bodies into the bath to a limiteddistance below the top surface thereof, and progressively mov ing saidbodies through the bath in a direction substantially parallel to theopposed electrodes.

4. The method of heating rod-like bodies such as rock drill tools,lwhich includes electrically heating a bath of molten salt by the directpassage of current transversely therethrough, the volume of currentpassing through the bath being greater at one end than at the other tomaintain a higher temperature at one end than at the other, immersingthe ends of said bodies into said bath, and progressively moving saidbodies through said bath from the portion at lower temperature to theportion at higher temperature while maintaining the degree of immersionof said ends substantially constant.

5. The method of heating rod-like bodies such as rock drill tools, whichincludes maintaining a heated bath of molten salt by the direct passageof current transversely therethrough, the volume of current passingthrough the bath being greater at one end than atthe other to maintain ahigher temperature at one end than at the other, immersing the ends ofsaid bodies into said bath, yprogressively moving said bodies throughsaid bath from the portion at lower temperature to the portion at highertemperature while maintaining the degree of immersion of said endssubstantially constant, and inhibiting the flow of convexion currentsin' said bath from one end toward the other.

6. The method of heating the ends of rod-like bodies such as rock drillsby means of a salt bath furnace heated by the flow of electric currentthrough the bath material between spaced electrodes, which methodincludes progressively and continuously moving a series of said bodiesalong a path lengthwise of the bath and transverse to the direction ofcurrent flow, holding the ends of said bodies at a short distance abovethe bath along the initial portion of the path Vto preheat the ends bythe gases from said bath, and maindetermined distance .while movingalong another portion of said path.

7. The method of heating the cutting ends of a series of rock drilltools each having a central passage, which includes maintaining a moltensalt bath, moving said tools lengthwise of said bath, maintaining thecutting ends at a short distance above the level of the bath along aportion of said path to permit heating and expansion of the air withinthe central passages of the tools, and maintaining the cutting endsimmersed to a predetermined distance in the bath during a furtherportion of the path of movement.

8. A salt bath furnace comprising a chamber for the bath material,spaced electrodes for heating said material. by the passage of anelectric current between electrodes and through the lntervening bathmaterial, and a conveyor arranged for moving a series of articles to beheated through the bath in a path following substantially anequipotential line between the electrodes.

9. A salt bath furnace comprising a chamber for the bath material,electrodes upon opposite sides of said chamber and spaced apart to adistance but little greater than the width of the articles to be heated,and a conveyor for moving the articles with portions only thereofimmersed in the bath and in a direction transverse to the flow ofcurrent between said electrodes.

10. A salt bath furnace comprising a chamber for the bath material,electrodes upon opposite sides thereof for heating the material bypassing an electric current transversely through the bath material, aconveyor above said chamber for moving the articles a short distancelengthwise of the bath and above the latter, and thence partiallyimmersing the articles in the bath and continuing the movement throughthe bath.

1l. A salt bath furnace having a chamber for the bath material,electrodes for heating the material by the passage of an electriccurrent therethrough, a pair of endless conveyors disposed in parallelvertical planes above the path, with horizontal runs adapted to supportarticles suspended therefrom and partially immersed in the bath.

l2. A salt bath furnace including a chamber for the bath material,electrodes for heating said material by the passage of an electriccurrent between said electrodes through the bath material, means forsupporting a series of rock drill tools with their lower ends immersedin said bath and for moving them lengthwise of the bath, the width ofsaid bath being only slightly greater than the width of the portions ofsaid tools immersed therein.

13. An electrically heated salt bath furnace comprising a bath chamber,a conveyor arranged for vertically suspending a series of bodies such asrock drill tools and moving said bodies in a substantially horizontaldirection with their lower ends immersed in said bath, and a series ofspacers slidable along the top of said bath and substantially closingsaid chamber, said spacers being moved by said conveyor through contactof said bodies with said spacers.

14. A salt bath furnace including a bath chamber, a conveyor forvertically suspending a series of bodies such as rock .drill tools andmoving said series in a substantially horizontal direction with thelower endsof the bodies extending into the bath, an auxiliary reservoirand a weir over which thebath material may flow to said reservoir tomaintain a substantially constant level within said bath.

15. A salt bath furnace having a bath c'hamber, electrodes therein andupon opposite sides thereof, said electrodes being adjustable toward andfrom each other, and means for supporting a series of articles withtheirlower ends immersed in the bath and conveying them through the bathbetween and substantially parallel to said electrodes.

16. A salt bath furnace comprising a bath chamber, electrodes uponopposite sides thereof, said electrodes being wider at one end of thebath than at the other, and means for supporting a. series of articleswith their lower ends extending into said bath and conveying themlengthwise of the bath from a point adjacent to the narrower end of saidelectrodes to a point adjacent to the Wider end of said electrodeswhereby the bath and. articles are progressively heated to highertemperatures.

17. A salt bath furnace comprising a comparatively narrow long bathchamber having electrode's upon opposite sides thereof, and means forvertically suspending a series of articles and advancing them lengthwiseof said chamber with their lower ends immersed in the bath. said chamberhaving an auxiliary reservoir portion adjacent to the bath level wherebythe area of the surface of the bath is substantially larger than theportion of the bath along the path of travel of said articles.

18. A salt bath furnace comprising a bath chamber having a plurality ofelectrodes upon opposite sides thereof, the electrodes adjacent to oneend of the bath being of larger surface area than those adjacent to theopposite end of the bath, and means for conveying a series of articlesthrough said bath from the end having the smaller electrodes to the endhaving the larger electrodes.

19. A salt bath furnace having a comparatively narrow bath chamber,electrodes presenting increasing surface contact with the molten saltalong the length of the bath from one end to the other, and means insaid bath for inhibiting transfer of molten salt from one section of thebath to another.

20. The method of heating the ends of hollow rod-like bodies such asrock drills by means of a salt bath furnace heated by the flow ofelectric current through the bath material between spaced electrodes,which method includes progressively and continuously moving a series ofsaid bodies along a path lengthwise of the bath, holding the ends ofsaid bodies at a short distance above the bath along the initial portionof the path to preheat the ends by heat radiated from said bath, andmaintaining said ends immersed in the bath to a predetermined distancewhile moving along another portion of said path, the rate of movement ofsaid bodies and the length of said initial portion of said path beingsuch that the air within the lower ends of said bodies becomes heated toa high temperature, greatly expanded, and flows out before said ends areimmersed, whereby the further expansion and upow ofl air in said bodiesafter immersion. does not suck molten liquid from said bath up into thebody, to a level substantially above the level of the bath.

JOSEPH DARIUS GRIFFIN.

